The project will be directed toward a genetic and biochemical analysis of the branched-chain amino acids in Escherichia coli and Salmonella typhimurium. The effects of gene mutation and physiological manipulation on the normal, multivalent repression pattern controlling 4 of the 5 biosynthetic enzymes leading to isoleucine and valine in whole cells will be examined in terms of enzyme activity and amount and in terms of mRNA levels and stability. The transcription and translation in vitro directed by ilv DNA will be studied in an effort to account biochemically for the roles of the regulatory elements discovered during in vivo experiments.